1. Is it ok to slow down by manually selecting third gear? ie will it wear my gearbox out over time? I hope not since I like doing it! I've asked a few people and the general consensus seems to be YES, but not based any real scientific-like evidence. Y'know, just the vibe of the thing :)
I saw the same question in this thread, but it has the same lack of conrete evidence.
Can anyone give me some concrete?
I figured it'd be ok since most of the time the box is going to change back to third once I start to accelerate again anyway. Of course, I know 100% of jack shit about automatic transmissions!

2. When cruising at 110k's, the engine does about 2300rpm. Is that right? I would have expected a v8 to rev lower, since my old XH ute did around the same and it was a six. I'm guessing 2300 is right, but I'm just curious as to why exactly.

Just leave it in 3 around town and D when highway cruising. The gear position sensor is in the neutral saftey switch, this unit is made of plastic and tin, I don't think the design is robust enough for "manual" shifting (for any extended period, anyway).

My NF V8 does about 1700 rpm at 100k's, 2300 at 110 sounds a tad high.

Might want to check if your torque converter is locking on. If you work out, 2300rpm on a 3.45 would be 2153rpm on a 3.23, TopGhia says he gets 1700 ... hope that gives you some idea. So yeh, I'd say 2300 rpm is a bit high. An easy test, on a flat road with little wind, try accelerate gently to eighty, the tacho should hold up about 2 grand whilst accelerating, then when you ease off to cruise at 80, the revs should drop down a fair bit as the torque converter locks on. If they stay high, might want to check out the torque conv system?

I don't know what evidence you are looking for, but you can rest assured dropping a gear while slowing down will wear out the box quicker.
Reasons:
1. the transmission was never designed to do it, it will not have any form of beefed up clutches/bands to deal with the load from the factory, so the torque your putting on the stock components will wear them much faster.

2. Heat. The increase in friction through the transmission will no doubt be heating your oil beyond the normal working temp of the box, especially in traffic. Without an aftermarket cooler added on it'll cook itself soon enough.

Why would you want to use a $3000 auto to slow the car when its got a braking system more than capable of pulling the car up? Brake pads only cost $100 to replace.

As far as your RPM goes it sounds about right to me if you have a 3.45:1 diff, no matter how many cylinders an engine has, the RPM's stay the same.